Neutrinos are arguably the most fascinating elementary particle in our universe. In cosmology they play an important role in the formation of large-scale structures, while in particle physics their tiny but non-zero mass sets them apart, pointing to new physics phenomena beyond our current theories. Without a measurement of the mass scale of neutrinos, our understanding of the universe will remain incomplete. Scientists often refer to the neutrino as the “ghost particle” because they almost never interact with other matter.
This is the
challenge the international Karlsruhe Tritium Neutrino
(KATRIN) experiment at Karlsruhe Institute of Technology (KIT) with partners
from six countries has taken up as the world´s most sensitive scale for
neutrinos. It makes use of the beta decay of tritium, an unstable hydrogen
isotope, to determine the mass of the neutrino via the energy distribution of
electrons released in the decay process. This necessitates a major
technological effort: the 70-meter-long experiment houses the world´s most
intense tritium source as well as a giant spectrometer to measure the energy of
decay electrons with unprecedented precision.
The high quality
of the data after starting scientific measurements in 2019 has continuously
been improved over the last two years. “KATRIN is an experiment with the
highest technological requirements and is now running like perfect clockwork”
enthuses Guido Drexlin (KIT), the project leader and one of the two
co-spokespersons of the experiment. Christian Weinheimer (University of Münster),
the other co-spokesperson, adds that “the increase of the signal rate and the
reduction of background rate were decisive for the new result.”
The 70 meter long KATRIN experiment with its main components tritium source, main spectrometer and detector. Credit: Leonard Köllenberger/KATRIN Collaboration
Data analysis
The in-depth
analysis of this data was demanding everything from the international analysis
team led by its two coordinators, Susanne Mertens (Max Planck Institute for
Physics and TU Munich) and Magnus Schlösser (KIT). Each and every effect, no
matter how small, had to be investigated in detail. “Only by this laborious and
intricate method, we were able to exclude a systematic bias of our result due to
distorting processes. We are particularly proud of our analysis team which
successfully took up this huge challenge with great commitment,” the two
analysis coordinators are pleased to report.
The experimental
data from the first year of measurements and the modeling based on a
vanishingly small neutrino mass match perfectly: from this, a new upper limit
on the neutrino mass of 0.8 eV can be determined (Nature Physics, July
2021). This is the first time that a direct neutrino mass experiment has
entered the cosmologically and particle-physically important sub-eV mass range,
where the fundamental mass scale of neutrinos is suspected to be. “The particle
physics community is excited that the 1-eV-barrier has been broken by KATRIN,”
comments neutrino expert John Wilkerson (the University of North Carolina, Chair of
the Executive Board).
Susanne Mertens
explains the path to the new record: “Our team at the MPP in Munich has
developed a new analysis method for KATRIN that is specially optimized for the
requirements of this high-precision measurement. This strategy has been
successfully used for past and current results. My group is highly motivated:
We will continue to meet the future challenges of KATRIN analysis with new
creative ideas and meticulous accuracy.”
Further measurements should improve sensitivity
The
co-spokespersons and analysis coordinators of KATRIN are very optimistic about
the future: “Further measurements of the neutrino mass will continue until the
end of 2024. To realize the full potential of this unique experiment, we will
not only steadily increase the statistics of signal events, we are continuously
developing and installing improvements to further lower the background rate.”
The development of
a new detector system (TRISTAN) plays a specific role in this, allowing KATRIN
from 2025 on to embark on a search for “sterile” neutrinos with masses in the
kiloelectronvolt-range, a candidate for the mysterious dark matter in the
cosmos that has already manifested itself in many astrophysical and
cosmological observations, but whose particle-physical nature is still unknown.